Second order ferro-paraelastic phase transition in the layer crystal (n-C3H7NH3)2 ZnCl4 at 310 K

Abstract
Bis-propylammonium tetrachloro-zincate (II) shows a second order ferro-paraelastic phase transition with a Pnma-P21/n symmetry change at 310.20±0.02 K. Experimental studies involving the description of both structures from x-ray measurements as well as thermal and optical measurements allow us to characterize this phase transition. It is equitranslational and is described by means of an order parameter which transforms according to the irreducible representation B3g of space group Pnma. The critical behavior of the spontaneous monoclinic shear u4, the spontaneous birefringence and the specific heat suggest that this compound may belong to the group of ’’proper ferroelastics’’ for which the Landau theory is strictly valid. A soft mode discussion that accounts for the displacive part of this phase transition and a phenomenological approach following Landau’s theory are carried out. Finally, a possible order–disorder contribution is also discussed.

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